1. Field of the Invention
This invention relates to integrated circuit packaging, and more particularly to wire bonding in a lead frame assembly.
2. Background of the Technical Art
The term "integrated circuit" usually refers to an integrated circuit die and the packaging associated with the die. A common way to package an integrated circuit die is to attach the die to a lead frame and encapsulate the structure in plastic. A typical lead frame includes a die attach pad and a number of bonding fingers that are electrically connected to the die by bonding wires.
Integrated circuit dies are designed with ever-smaller feature sizes to increase performance and to reduce the cost of an individual integrated circuit. An integrated circuit die with smaller feature sizes will often have smaller dimensions than a functionally equivalent integrated circuit die having larger feature sizes. Although integrated circuit die sizes are becoming smaller, integrated circuit package sizes are commonly fixed by standards and conventions adopted by the electronics industry.
When small die are packaged within lead frames designed for larger die, the manufacturing yield of the packaged die often suffers. Because a die is physically smaller than optimum for that package, longer bonding wires are required to electrically connect the die to the bonding fingers on the lead frame. These longer bonding wires are often quite flexible and can short out through contact with adjacent bonding wires. This may occur through an intrinsic twist of the bonding wires or because of forces developed during the encapsulation process.
One solution to this problem, i.e. making lead frames that accommodate smaller die sizes, is expensive. The bonding fingers of the lead frame become so small and so closely spaced that the lead frames must be formed by an expensive etching process, rather than by an economical stamping process.
Another solution to this problem is to use what is known as an interposer. A plurality of intermediate bonding islands are provided on the die attach pad of the lead frame so that the die can be electrically connected to the corresponding bonding wire in two or more jumps through intermediate or interposed connections at the bonding islands. The bonding pads of the die are wired to bonding islands of an interposer with a first (inner) set of wires. The bonding islands are then wired to the bonding fingers of the lead frame with a second (outer) set of wires. By making the electrical connections between the die and the bonding fingers in two or more jumps, the longer direct connections are replaced by two or more sets of shorter wires that are less likely to experience shorting than are longer connections.
Interposers in the prior art are usually small printed circuit boards having a relatively thick, rigid, insulating substrate upon which the conductive islands are formed. The printed circuit boards are attached to the die attach pads of the lead frame and are provided with central apertures to allow attachment of an integrated circuit die through interposers to the die attach pads.
While interposers perform their function admirably, they do have some disadvantages. First, interposers now available are relatively expensive because they are essentially miniature printed circuit boards. Second, because the size of a die aperture is fixed, a particular interposer can only be used for a small range of die sizes. An interposer cannot be used when the die size is larger than the diameter of the die aperture. Third, if an interposer is used with a die whose size is much smaller than the size of the die aperture, the inner set of wires must be made longer, which reduces the effectiveness of the interposer. Fourth, the bonding islands have intrinsic inductances, which can degrade high frequency performance of the associated integrated circuits.
What is needed is an interposer technique that allows fabrication of interposers at lower cost and with more control of electrical inductances and relevant dimensions of these interposers.